Electromagnetic data storage devices are well known, and are a convenient and reliable way to store large amounts of electronic data. The most common type of electromagnetic data storage device is the hard disk drive (HDD) device. An example HDD 1 is shown schematically in FIG. 1 of the accompanying drawings, and includes a spinning disk 2 which carries magnetic material for storing data, a write head 4 for affecting the magnetization direction of a selected portion of the magnetic material, and a read head 4 for detecting the magnetization direction of the portions of the magnetic material in the disk. The write and read heads 4 are typically located on a single actuator device 5 which operates to move the heads 4 across the surface of the disk 2. The heads 4 do not contact the surface of the disk 2, but are separated for the surface by a small air gap caused by air moving with the disk as the disk spins. A detailed explanation of the construction and operation of a hard disk drive device will not be included here for the sake of brevity, since such construction and operation are well known.
There is a continual demand for ever higher storage densities on hard disk drive devices. Such demand is driven, for example, by the increasing desire for use of data-intensive multimedia and video applications on ever smaller devices.
Other mass storage systems with no moving parts, predominantly flash memory, based on non-volatile electronic storage, are available but HDDs remain the main medium for secondary storage on computers and other devices. One reason is the durability of HDD storage to the number of read/write cycles. Data can be written, erased, rewritten over and over indefinitely as long as the disk isn't damaged. All other technologies have a finite number of read/write cycles that guarantee reliable storage. Despite the extra mechanical complication in having a rotating disk within a device, expected improvements in storage density and speed will make HDDs the dominant technology for several years ahead.
The most fundamental limit to data storage on a magnetic data storage medium is the so-called superparamagnetic limit. A piece of magnetic material that is below a critical size (typically around 10 nm) will not hold a permanent magnetization at room temperature. A nanoparticle that is below this critical size is termed superparamagnetic and cannot be permanently magnetized at room temperature. The critical size depends on the magnetic anisotropy of the nanoparticle and decreases with increasing anisotropy. The magnetic film that stores data on an HDD is composed of nanoscale particles and a single data bit is written onto a region containing typically 100 nanoparticles. This is illustrated in FIG. 2 of the accompanying drawings for a single ‘1’ bit written into the magnetic medium of a high-density HDD, which consists of a magnetization reversal. The left part of the Figure shows a measurement of the magnetization pattern on the disk measured by an atomic force microscope and the ‘1’ is highlighted occupying a region of around 100 nm×100 nm. The right part is an illustration of the film that stores the data, consisting of nanoparticles with sizes around 10-20 nm.
In order to increase the density further, this size of the region storing the bits must be reduced and this leads to an impasse. Looking at the possible solutions:                Magnetize a smaller region. This is not possible as reducing the number of particles within a single bit will increase the read back noise. Already the area includes the smallest number of particles that produce an acceptable signal to noise ratio.        Use smaller nanoparticles to decrease the size of the region. This is not possible as the particles will be superparamagnetic.        Use smaller nanoparticles with a higher anisotropy. This is not possible because a higher write field is required to magnetize the medium. Already the highest possible write field is used by flying the write head as close as possible to the medium (12 nm) and the most magnetic material available (FeCo alloy) is coated on the tip of the write head.        
Accordingly, the inventors of the current invention have concluded that storage density can be increased if the magnetic field used to write to the magnetic storage material could be increased.
Magnetic materials find widespread use in modern technology and are to be found in nearly all electro-mechanical apparatuses. The performance of magnetic materials in respect of their secondary parameters, such as coercivity and energy product, has improved greatly over the last century. There has nevertheless been little improvement in the most fundamental property, i.e. the saturation magnetization, which determines the strength of the magnetic field produced. The most magnetic material for use in electro-mechanical apparatus, i.e., Fe60Co40 alloy, has been available since the 1920s and until recently there has been no material found with a higher magnetization.
The most direct measure of saturation magnetization is the magnetic moment per atom which is specified in Bohr magnetons (μB). The magnetic moment for pure Fe is 2.22μB per atom whereas for Fe60Co40 alloy the magnetic moment is 2.45μB per atom. The latter value, i.e., 2.45μB per atom, is termed the Slater-Pauling limit and was believed to be the ultimate magnetization available from transition metal alloys. Generally the efficiency of electro-mechanical apparatus improves as the square of the magnetization of the magnetic material. Even small increases in magnetization are therefore valuable especially in green technologies such as electric vehicles and wind turbines.
Upon development in the early 1990s of gas-phase nanoparticle sources capable of depositing nanoparticles with diameters in the range of 0.5 to 5 nm it was discovered that the magnetic moments per atom of Fe, Co and Ni nanoparticles with diameters no more than about 5 nm are significantly higher than for bulk structures formed from the same material. In view of this, magnetic structures in which nanoparticles of one of Fe and Co are embedded in a matrix of the other of Fe and Co have been developed with such magnetic structures having a magnetization which exceeds the magnetization of Fe60Co40 alloy to thereby break the Slater-Pauling limit for the first time. FIG. 3 illustrates the formation of one such magnetic structure. As shown in FIG. 3 a magnetic structure 10 is formed by co-deposition on a substrate 12 of Fe nanoparticles 14 from a cluster source 16 and of Co matrix material 18 from a Molecular Beam Epitaxy (MBE) source 20. Co-deposition of Fe nanoparticles and Co matrix material results in a structure in which Fe nanoparticles are distributed through and embedded in the Co matrix. According to an alternative approach a magnetic structure in which Co nanoparticles are distributed through and embedded in an Fe matrix is formed by co-deposition of Co nanoparticles from the cluster source and of Fe matrix material from the MBE source.
Respective magnetic moment per atom measurements for a structure having Fe nanoparticles in a Co matrix and a structure having Co nanoparticles in an Fe matrix are shown in FIG. 4 as a function of the Fe volume fraction. FIG. 4 also shows the Slater-Pauling curve for Fe60Co40 alloy as a function of the Fe volume fraction. As can be seen from FIG. 4 the best results are obtained from Co nanoparticles embedded in an Fe matrix which yields values approaching 3 pB per atom. At lower Fe volume fractions the magnetic moment per atom for Fe nanoparticles embedded in a Co matrix exceeds the corresponding value defined by the Slater-Pauling curve. The improvement is seen because the fundamental building blocks of the material already have an enhanced magnetization and also because the matrix itself has a nanostructure which leads to enhanced moments. More specifically there is a higher proportion of atoms at a surface or interface in a nanostructure (approaching 50% in the presently described structure) with each such atom having enhanced spin and orbital moments. On the other hand and as can be seen from the left half of the graph of FIG. 4 the magnetization falls below the Slater-Pauling curve at Fe volume fractions of more than about 20% which is the percolation threshold.
Accordingly, it is desirable to provide a process that can deliver a material with improved magnetic characteristics for use in electromagnetic data storage devices. It is also desirable to provide an electromagnetic data storage device that makes use of such a material.